A Mechanism-Based Strategy for Controlling CH4 and CO Selectivities in CO2–H2 Reactions on Dispersed Ru, Co, and Ni Nanoparticles

Abstract

The formation of CO as an intermediate in the conversion of CO₂–H₂ to CH₄ and its strong binding on dispersed Ru, Co, and Ni nanoparticles inhibit rates of CO and CH₄ formation but to different extents. CO₂ conversion rates decrease and CH₄ selectivities increase as CO concentration gradients evolve axially along the catalyst bed and radially within diffusion-limited porous aggregates. These trends and their interpretation in terms of the identity and kinetic relevance of surface-catalyzed elementary steps enable mechanism-based strategies for selectivity control through the purposeful introduction of CO pressures into inlet CO₂–H₂ streams. This strategy exploits the stronger CO inhibition of its formation (from CO₂) than its conversion (to CH₄), which causes the selective inhibition of CO₂ conversion relative to CH₄ formation. The presence of CO at levels accurately prescribed by the mechanism-derived rate equations, similar in functional form on Ru, Co, and Ni nanoparticles, and by diffusion-convection-reaction models that account for CO gradients at the bed and aggregate scales led to the exclusive formation of CH₄ and to the elimination of CO gradients at both scales, as evident from measured rates and selectivities for CO₂–H₂ reactions on Ru, Co, and Ni nanoparticles over a broad and practical range of temperature (483–573 K), reactant pressures (4–1100 kPa CO₂; 8–820 kPa H₂), and nanoparticle diameter (2–30 nm). This mechanism-based strategy enables the exclusive formation of CH₄ from CO₂–H₂ reactants, irrespective of reaction conditions or nanoparticle composition (Ru, Co, Ni) and size, without requiring complex catalyst architectures or intricate synthesis protocols.